[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Sep 6 08:56:24 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT WED SEP 6, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

The strong upper-level ridge responsible for the recent heat-wave is slowly progressing eastward.  In addition to wildfire smoke, increasing southerly flow aloft is transporting mid-level moisture northward over the state, from a decaying tropical system off the northern California coast.  A surface thermal trough remains over the northern Willamette Valley this morning, with weak onshore flow from Newport to Salem and continued offshore flow from Redmond to Salem.

The surface thermal trough is expected to push east of the Cascades this afternoon with increasing onshore flow helping the clear low-level wildfire smoke from the Willamette Valley.  However, south-to-SE flow aloft will continue to force elevated wildfire smoke over the region, from SW Oregon and northern California, and enough moisture and instability for possible thunderstorm development.

TODAY'S FORECAST:

Mostly cloudy and smoky.  Not as hot.  Chance of afternoon and evening thunderstorms.  Any rainfall totals likely less than .10"

Salem's high temperature today will be near 86°F (average is 80°F).

Relative humidity:  Dropping to 50% around noon and to 45% by 5 p.m.

Surface winds: S 5 mph this morning; SW 5 mph this afternoon.

Transport winds: S 5 mph this morning; SW 5-10 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to 3500 feet by 5 p.m.

Salem's sunset tonight: 7:39 p.m.

(Salem Airport data for Tuesday, September 5th: High 89°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 35)

EXTENDED DISCUSSION:

An increasing unstable southerly flow aloft will continue on Thursday, as increasing onshore surface winds cool temperatures back close to average and purge low-level smoke from the valley.  Once again, thundershower development is possible but with a greater chance of more significant rainfall, especially near the Cascades.

The flow aloft may turn more southwesterly on Friday, which would push the threat of thunderstorms east of the Cascades.  Continued onshore flow will hold temperatures near average and could create a burning opportunity.

The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text

Notes:

     1.  Mixing height, as used here, is the lowest height at which the

         potential temperature exceeds the potential temperature at the

         surface.  As a practical matter it is the approximate height to

         which a smoke plume will rise assuming good ignition, dry fuels,

         and winds less than about 15 mph.

     2.  Transport winds are a layer average through the mixing height,

         weighted slightly toward the winds at the top of the layer.

     3.  Ventilation Index is the height of the mixing layer (ft) times

         the transport wind speed (mph) divided by 1000.

     4.  Surface wind direction is the general expected wind direction.

         At a specific point surface winds are highly dependent on local

         terrain conditions.

     5. In accordance with OAR 837-110-0090, all field burning shall be
         banned when any two of the following criteria are present:
         A. Temperature of 95 degrees Fahrenheit or above
         B. Relative humidity of 30 percent or below
         C. Wind speed of 15 miles per hour or higher

This forecast is provided under an agreement between the Oregon Department

of Agriculture (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.

To add/remove your email address from this list, please go to:

http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst

Pete Parsons

ODF Meteorologist

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